The present invention pertains to airfeed drills; more particularly, the device of the present invention relates to airfeed peck drills.
An airfeed peck drill is, in its simplest terms, an air driven rotary drill mounted on the front of an air driven power cylinder. The features of a peck drill which distinguish it from a continuous feed drill are the short period of contact of the drill with the workpiece, the repeated complete extraction of the drill from the hole being drilled and the repeated re-insertion of the drill into the hole being drilled. Drill travel speed into the workpiece is controlled by mechanical contact of the moving drill with the outboard end of a piston rod extending from a very accurate damping cylinder. A controlled leak around the piston in the damping cylinder enables the damping cylinder to provide a continuous moving counter-force to the force used to move the airfeed peck drill into the workpiece. In order for the drill travel speed control to remember the position of the drill while the airfeed peck drill is extracted and returned to the bottom of the hole being drilled, it is necessary to prevent rearward travel of the piston rod while the face of the drill bit is not in contact with the workpiece. The rearward travel of the piston rod is prevented by a device which has become known as a set-back control device. Further explanation of set-back control in airfeed peck drills may be found in U.S. Pat. Nos. 4,123,188 and 4,421,441.
The accurate control of airfeed peck drill travel is necessary to prevent improper drilling of holes or drill bit damage. Improper hole drilling or damage to drill bits usually occurs when a drill bit reengages material at the bottom of a hole under uncontrolled feed rate conditions. Such uncontrolled feeds normally occur just at the beginning of the compression of the fluid in the damping cylinder and during the period while the drill frame is loaded by thrust from the feed piston. Additionally, as the rotating drill contacts the workpiece, it causes both the fixture and the workpiece to deflect. When the rotating drill moves away from the fixture and the workpiece, the fixture and the workpiece return to an undeflected or relaxed condition. As the rotating drill moves back into the workpiece and contacts the bottom of the hole, the force of the drill tends to cause the fixture and the workpiece to separate. It is necessary that as force is placed against the bottom of the partially drilled hole that this is done under controlled feed conditions. Without adequate set back, this cannot be accomplished. To overcome these conditions, sufficient rearward movement or set back is required to allow the drill feed control mechanism to compensate for the periods of uncontrolled drill feed and fixture and workpiece deflection.
U.S. Pat. No. 3,680,970 to Deschner teaches an airfeed peck drill set back control device which has become a standard in the industry. This airfeed peck drill set back control device has two drawbacks. First, it is not adjustable, that is it cannot be made to compensate for differing periods of uncontrolled feed of fixture deflection allowed by the drill feed control mechanism and second it has a large extension which hangs off one side of the piston rod. This extension makes any airfeed peck drill incorporating this device difficult to use in confined areas.
There is therefore a need in the art to provide a set back control device which is adjustable and is smaller in size.